Controllers, including indicating controllers, are widely used in process control systems to control one or more process variables such as pressure, flow, temperature, liquid level, or the like. In a typical system the controlled variable is measured and the measured value is fed back for comparison with a reference input sp comprising the desired value of the controlled variable. An error signal, E, comprising the difference between the reference input sp (or set point) and measured value of the controlled variable, is supplied as an input to controlling circuits such as dead zone comparators, proportional gain amplifiers or multipliers, integral and derivative compensation circuits, and the like, for the desired controller action. A three mode controller employs the three proportional, integral and derivative control actions. The output (i.e. manipulated variable) from the controlling circuit or circuits is supplied to the controlled system to maintain the controlled variable at the set point. Digital, analog or a combination of digital and analog circuitry may be employed.
Prior art controllers often include one or more analog meters at the front panel thereof for display of one or more controlled values such as the measured value of the controlled variable and the error signal. A dial for setting and indicating the set point also is included at the panel. Various other control knobs for mode adjustment are included, which often are located behind the front panel to reduce the already cluttered appearance of the front panel. Generally, no display of the mode adjustments other than the physical positioning of the knobs with respect to their associated dials is provided. Actual values of many directly adjustable and non-adjustable controller circuit parameters can be checked only by use of suitable test instruments.